Pollination Services Provided by Bees

Pollination, the movement of genetic material in the form of pollen grains, is a key step in the development of most food crops. Even crops that do not rely on insect pollination - wind pollinated or self-pollinated crops -are sometimes more productive when visited by an insect pollinator. Bees are a particularly important group ofinsect pollinators, responsible for pollinating 60-70% of the world's total flowering plant species, including nearly 900 food crops worldwide, such as apples, avocados,

Figure 1 Joint products of ecosystems. Many ecosystems are currently managed to exploit only one service. Managing for multiple services can increase ecosystem benefits.

cucumbers, and squash. These crops comprise 15-30% of the world's food production, and bees are credited with $4.2 billion in annual crop productivity in California alone. Bees are especially important pollen vectors in part because physical adaptations, such as hairs designed to pick up pollen, and behavioral adaptations, such as fidelity to a single species of plant on each pollen-gathering trip, ensure good pollen transport and cross-pollination.

In the US, most major agricultural enterprises that rely on bee pollination import managed bees, almost always the European honeybee Apis mellifera. The available stock of managed honeybees has declined dramatically, however, dropping by over 50% in the last 50 years, while demand for pollination services has increased in many areas. This decline in managed bee populations has many causes, including increased pesticide use, disease in the hives, and downsizing of stocks that have hybridized with Africanized bees, introducing traits that make managed bees more aggressive and thus a liability to the farmer.

The contribution of native, wild bees to agricultural pollination was ignored, and assumed to be negligible, until the early 2000s. Since then, research has shown that native bees serve an important role in pollination, picking up slack when managed bee pollination is insufficient and enhancing crop production in general. Farms with generous native bee habitat nearby may be able to fully or partially replace pollination by managed bees. In some cases, native bees are more efficient pollinators than European honeybees. The variety of wild bees, with distinct physical and behavioral traits, allows them, as a group, to pollinate a wide variety of flowering plants. Tomatoes, for example, have pollen that is accessible only by vibrating the flower, which bumble bees and some other native bees can, while honeybees cannot. Though tomatoes are self-pollinating and do not require an insect vector, native bees promote cross-pollination, which, for example, significantly increases the fruit set and size of Sungold cherry tomatoes.

The contributions of native bees to crop production are usually undocumented and underestimated, and they are always unpaid, at least directly. Though hives of managed honeybees must be rented or maintained, wild bees pollinate at no cost to the farmer. Populations of native bees are under great threat, however, by land management practices that promote the use of pesticides and the loss, fragmentation, and degradation of habitat. Protecting native bees without protecting the ecosystems in which they live is impossible. Native habitat, unlike agricultural monocropping, provides the year-round supply of blooming plants that wild bees require for sustenance. Native habitat also provides nesting areas; most wild bees are solitary, laying a single egg in a nest cavity dug into the ground or into dead wood, not forming social hives. In order to reap the benefits of native pollinators, food resources and nesting habitat must be available within a short distance of crops, possibly as hedgerows, in ditches, or around water ponds. A study of wild bee pollination of coffee in Costa Rica showed that farms closer to tropical forest remnants were visited by many more species of wild bees than those further away. Had the far sites been adequately pollinated, coffee yield would have been increased by nearly 20% and misshapen coffee beans reduced by 27%. A lower-bound estimate of the pollination services from these patches is US $62 000 per year (in the early 2000s).

The diversity of the native bee population is one of its strengths. Many species of bees participate in pollination, and the abundance of different species varies year by year. This diversity allows the native pollinator community to be both resistant, maintaining functionality in the face of environmental upheaval, and resilient, able to reestablish itself in the wake of a destructive event. When the population of Apis declined dramatically in the second year of the Costa Rica study, sites close to forest fragments showed minimal loss of pollination while pollinator visits dropped by nearly 50% further away. Thus, as well as enhancing pollination services in conjunction with managed bees, native bee populations provide important insurance against the possibility that managed bee populations could fail because of disease, hybridization, or other causes.